1. Field of the Invention
The present invention relates to new ammoxidation catalysts for the production of unsaturated nitriles starting from their corresponding olefins, in particular, for the production of acrylonitrile from propylene. More specifically, the present invention is directed to an improved ammoxidation catalyst containing niobium and silver as essential elements for enhancing the activity and selectivity of the catalyst system.
2. Description of Related Art
Several publications are referenced in this application. The references describe the state of the art to which this invention pertains and are hereby incorporated by reference.
A number of ammoxidation catalysts are known in this field such as those disclosed in U.S. Pat. Nos. 4,405,498; 5,688,739; 4,600,541 and European Patent Publication Nos. 00 32 012 B1; 05 73 713 B1; 04 75 351 A1. All of these publications relate to molybdenum catalyst systems.
It is known in the art that the bismuth-molybdenum system plays a role of electron donor/acceptor mechanisms for selective oxidation and ammoxidation. Therefore different mechanisms were proposed based on this property [Delmon et al. (New Development in Selective Oxidation by Heterogeneous Catalysis, Vol. 72, 1992, p. 399-413) and Encyclopedia of Chemical Technology (Kirk-Othmer, Vol. 1, 4th edition, page 358)]. In these mechanisms, molybdenum was shown to be responsible for oxygen and nitrogen uptake and insertion into the substrate, while bismuth plays the role of H-abstraction of the methyl group in the .beta. position. Therefore, bismuth and molybdenum should be present on the catalyst surface in adjacent form in order to form the suitable active phase for this reaction.
It should be noted that a deficiency of bismuth on the catalyst surface leads to total oxidation reactions of the substrate.
It is also well known that antimony plays a role of a donor and thus could improve the selectivity of a catalytic system. Antimony can also play an additional role by isolating the vanadium active centers which are highly active towards oxidation reactions. This leads to minimizing the total oxidation reaction and directs the reaction towards the desired product.
Many catalysts have been disclosed for the foregoing reactions. One such catalyst is described in U.S. Pat. No. 4,062,885, where BiMoSbV systems were used as active elements. The catalyst was used for the preparation of phthalonitrile by the ammoxidation of ortho-xylene. The use of such catalysts for oxidation or ammoxidation reactions involving unsaturated aliphatic hydrocarbon is not mentioned.
U.S. Pat. No. 4,040,978 relates to a catalyst for a ammoxidation reaction containing bismuth molybdate mixed with other elements.
U.S. Pat. No. 4,405,498 relates to a catalyst for oxidation and ammoxidation reactions containing BiMoVSb with additional elements of groups IA, IIA, IVA, VA, VIA, IB, IVB and VIIB of the periodic Table of the Elements. The patent does not disclose the use of niobium. Although, silver was disclosed, experimental results for the catalyst with silver did not reflect any performance improvement.
U.S. Pat. No. 4,600,541 relates to a catalyst comprising FeBiMo and promoters such as Pd, Pt, Os and Ir.
More recently, European Patent Publication No. 0 475 351 A1 relates to a catalyst containing KFeSbMo which could be promoted by Nb and W. The best yield was achieved with a catalyst of the formula Fe.sub.10 Sb.sub.10 Mo.sub.9 Bi.sub.2 K.sub.0.6 Ni.sub.5.5 W.sub.0.3 B.sub.O.75 P.sub.0.75 (SiO.sub.2).sub.70.
European Patent Publication No. 0 573 713 B1 relates to a catalyst comprising MoBiFeCoNiCr promoted with at least three other promoters of alkali metals, alkaline earth metals, rare earth metals, Nb, Tl and As, with Fe, Co, Ni and Cr as essential catalyst components.
U.S. Pat. No. 5,688,739 relates to a multi-component catalyst. The base of this catalyst is bismuth molybdenum. Germanium was added as an essential element.
None of the prior art references discloses or suggests catalysts which provide high performance for the selective production of unsaturated nitrites from their corresponding olefins. Accordingly, it would be desirable to produce an improved catalyst for use in the selective production of unsaturated nitrites from their corresponding olefins.